Left Hand Coordinates???

Included in the sample code project MigratingOpenGLCodeToMetal is the file AAPLMathUtilities. This file contains two functions for creating projection matrixes:

/// Constructs a symmetric perspective Projection Matrix
/// from left-handed Eye Coordinates to left-handed Clip Coordinates,
/// with a vertical viewing angle of fovyRadians, the specified aspect ratio,
/// and the provided absolute near and far Z distances from the eye.
matrix_float4x4 AAPL_SIMD_OVERLOAD matrix_perspective_left_hand(float fovyRadians, float aspect, float nearZ, float farZ);

/// Constructs a symmetric perspective Projection Matrix
/// from right-handed Eye Coordinates to left-handed Clip Coordinates,
/// with a vertical viewing angle of fovyRadians, the specified aspect ratio,
/// and the provided absolute near and far Z distances from the eye.
matrix_float4x4  AAPL_SIMD_OVERLOAD matrix_perspective_right_hand(float fovyRadians, float aspect, float nearZ, float farZ);

So, I guess one can define the geometry of one's objects using either a right hand or a left hand coordinate system and have it display properly depending on which of these two one uses to construct the projection matrix.

This is good. I think I understand things much better now.